To watch Steve Ramirez and Xu Liu, PhD,manipulate a mouse memory with a laserbeam, go to www.ted.com/talks/steve_ramirez_and_xu_liu_a_mouse_a_laser_beam_a_manipulated_memory.

Extinguishing the traumatic aspect of a memory involves
creating new, safer mental associations to the same sensory
cues. Even long-term memories, when recalled, have plasticity
and the potential to be updated, an ability psychologists
co-opt during exposure therapy, in which a patient faces
his or her fears in a non-threatening environment in the
hope of gaining control of them. This requires neural
communication among a number of areas in the brain: The
hippocampus cues the ventromedial prefrontal cortex of
changed conditions, which inhibits neuron activity — and
the conditioned fear response — in the amygdala (Annual
Review of Psychology, 2012).

In some people, though, the process goes awry, and they’re
unable to escape intrusive thoughts.

“We really don’t know why people respond so differently
to traumatic experiences,” says Gregory J. Quirk, PhD, who
investigates the neuroscience of fear at University of Puerto
Rico School of Medicine. “It may be that the prefrontal cortex is
less connected to the amygdala, so it can’t say, ‘No, you’re not in
danger right now.’”

Malleable memories

Some scientists are trying to manipulate the reconsolidation
process. Ramirez co-authored a 2014 study in which he and
a team from RIKEN-MIT Center for Neural Circuit Genetics
were able to change bad memories to good in male mice. Using
a technique called optogenetics, in which genetically encoded,
light-responsive proteins are inserted into cells, the scientists
were able to pinpoint where a mouse’s negative memory
of a shock to the foot was formed, in the neural circuitry
that connects the dentate gyrus in the hippocampus to the
amygdala. The researchers then manipulated those neurons
with lasers. Each time the mice ventured to a certain part of
their enclosure, the negative memory was reactivated, and they
quickly learned to fear the area.

The male mice were then allowed to frolic with female mice,
while the same neurons were tapped, helping to switch their
messages from one of pain to one of pleasure. The next time the
male mice ventured into the chamber, their fear reactions had
vanished (Nature, 2014).